The present invention relates to chemical vapor deposition, and, in particular, relates to apparatus of applying the chemicals to the substrate in question.
Chemical vapor deposition (CVD) is one of the methods of growing epitaxial layers on a substrate material.
In one prior CVD apparatus, the gaseous reactants are introduced into a quartz tube reactor vessel in which a substrate is held by a substrate holder and a susceptor. The susceptor is heated so that the reactants pyrolyze and deposit out onto the heated substrate thereon. The CVD apparatus includes a vapor delivery apparatus having the desired gas input into a series of stacked flat plates positioned about a slotted gas pipe or rod. The stacked plates surround the slotted gas rod and form a series of flat flow channels to deliver the gas. The type of gas input can be changed by valving into the vapor delivery apparatus. Because of the distance between the heated substrate and the vapor delivery apparatus, spent gas may recirculate affecting thickness and compositional uniformity so that graded layer transistions occur when changing gas types. This results in a poor or inoperative semiconductor structure.
One method of eliminating spent gases is to inject inert gases to clean the reactor vessel but thermal degradation of prior deposited layers can occur while the inert gas is flushing the reactor vessel. The thermal degradation of exposed surfaces of epitaxial layers also results in degraded crystal structures.
Another problem of prior apparatus is the pre-mature mixing or pre-reaction of the reactants before they reach the substrate.
A further problem with prior CVD reactors is non-uniform crystal layer growth due to variations in the reactant gases. Prior solutions to this problem have been a long entrance length and/or inserts that help produce non-turbulent flow.
Another problem with prior CVD reactors is that only a small portion of the reactants actually deposit on the substrate so that one must collect and dispose of the spent reactant gases which are typically toxic.
Other problems with prior apparatus involve heating the substrate for proper chemical bonding where the reactant gases react. Several methods have been used including resistive, infrared, inductive or convective heating. When the whole substrate is heated, uniformity of heating has been a problem.
These problems clearly affect the ability to produce high quality semiconductor components such as high electron mobility transistors and multi-quantum well laser diodes since they require extremely sharp heterojunctions between dissimilar layers of semiconductor crystals.
When heterojunctions are not sufficiently abrupt, grading and other defects occur which decrease component performance, reliability and life. Creating abrupt heterojunctions is a difficult requirement to consistently fulfill because many sophisticated heterojunction structures require abruptly switching of material in ten to twenty angstrom thick layers.